Digital Audio for NTSC Television
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Digital Audio for NTSC Television Craig C. Todd Dolby Laboratories San Francisco 0. Abstract A previously proposed method of adding a digital similarities between B-PAL and M-NTSC indicated carrier to the NTSC broadcast channel was found to that the Scandinavian test results would apply in the be marginally compatible with adjacent channel U.S. Our original 1987 proposal was: operation. The technique also has some problems unique to broadcasters. Digital transmission A. QPSK carrier with alpha=0.7 filtering. techniques are reviewed, and a new set of digital transmission parameters are developed which are B. Carrier frequency 4.85 MHz above video thought to be optimum for digital sound with NTSC carrier. television. C. Carrier level -20 dB with respect to peak vision carrier level. I. Introduction Compatibility testing of that system has been It is very feasible to compatibly add digital audio to performed, and some television sets have been found the NTSC television signal as carried on cable on which the data carrier causes detectable television systems. Besides the marketing advantage interference to the upper adjacent video channel in a which can accompany the use of anything "digital", clean laboratory setting. It should be noted that with there are some real advantages to digital these problem sets, the FM aural carrier also caused broadcasting, especially where the transmission path noticeable interference to the upper adjacent picture. is imperfect. Digital transmission is inherently The interference from data occurs into luminance, robust. While the coding of high quality audio into and manifests itself as additive noise between digital form theoretically entails a loss of quality, approximately 1 MHz and 1.4 MHz. The noise there is no further loss of quality when the digits are level in a problem TV is subjectively similar to the transmitted through an error-free channel. This is in noise level resulting from a video carrier-to-noise stark contrast to the transmission of analog audio, in ratio (CNR) of approximately 47 dB. While this which a "perfect" channel is required to avoid level of interference may not be detectable given the degradation. Unfortunately, the intercarrier sound current quality level of cable signals in consumers' channel used with broadcast television is somewhat homes, the potential of optical fibre to raise tlJ.e less than perfect, and inherently limits the sound quality level of cable TV warrants a reduction in the quality of the analog BTSC stereo system. level of interference by approximately 6 dB to be safe. Digital transmission techniques have matured to the point where they can be economically applied to The system should be compatible with broadcast 1 2 broadcast audio. Previous work • led to a proposal television as well as cable television. We must for the addition of digital audio to the NTSC consider broadcast problems which can result from television broadcast signal. That proposal borrowed existing transmitter plant, and broadcast spectrum heavily from work performed in Sweden3 and allocations. These considerations also call for some Finland where a 512 kb/s QPSK carrier was changes to the previously proposed system. extensively tested with PAL system B. The 50- 1990 NCTA TECHNICAL PAPERS II. Digital Modulation Basics bits/sec/Hz. Fig. 2 shows a block diagram of this modulator which is simply a pair of double sideband Figure 1 shows the spectrum of a random data AM modulators in quadrature. The double sideband stream with a data rate of 250k bits/sec. In a digital modulator can be considered to be a phase transmission the data is always intentionally modulator, generating phases of 0 and 180 degrees. scrambled so that the transmitted data appears The second modulator can be considered to generate random. Note the spectral nulls at multiples of the phases of 90 and -90 degrees. The combination of data rate (250 kHz, 500 kHz, etc.). From Nyquist the two phases will generate one of four phases, sampling theory we know that all information in this ±45 and ± 135 degrees. This form of modulation is signal may be gleaned from the first 125 kHz, i.e. typically called Quadrature Phase Shift Keying or QPSK. 0 Fig. 3 shows the constellation diagram for QPSK. There are four possible states of the RF carrier phase. We refer to a chosen state as a symbol. -10 Since there are four possible states per symbol we are transmitting two bits per symbol with a symbol ---c:l rate of 250k symbols/sec, for a total data rate of ~ -20 v ~ Q Axis :::=' Q.s -30 0 0 < -40 -50 '-----'----L.-....l--.L..-----L---L-....1.----1 I Axis 0 125 250 375 500 625 750 875 1000 Frequency (kHz) Figure 1 Spectrum of 250 kb/s data the Nyquist frequency which is IJ2 of the 250 kHz sample rate. If we brick wall filter this signal at the 0 0 Nyquist frequency of 125kHz and use it to modulate a carrier, we will have a double sideband modulated Figure 3 QPSK Constellation bandwidth of 250 kHz, which will carry 250k bits/sec of data (1 bit/sec/Hz). If we add a second 500k bits/sec. If the low pass filters were not similar channel at the same frequency but in present, the phase would be instantaneously jumping quadrature, we will double the information in the between the four states shown, at a rate of 250 kHz. 250 kHz bandwidth RF signal to 500k bits/sec, or 2 In practice the actual signal will be continuously traversing between these points and the receiver will I do:to. sample the signal at the time when the signal passes through the constellation points. Modulo. ted Co.rrier The QPSK receiver is shown in Fig. 4. A carrier recovery loop coherently regenerates the transmitted carrier, which was suppressed in the modulator. Q do.to. Quadrature mixers demodulate the I and Q signals, Figure 2 QPSK Modulator low pass filters limit the effective receive bandwidth, 1990 NCfA TECHNICAL PAPERS- 51 1 • [ O<w< ;y -«)] 2 H(jw) _ oos { ;; [w- ~(~«)]}, ~(1-cx)!>w!>~(1 +ex)] (1) [ Ts Ts 0, [.,, ;}1 +«)] excess bandwidth over a perfect brickwall (alpha==O) Eye filter. A filter meeting the amplitude response of Moni-tor Eq. 1, and having constant group delay (linear phase) will have no intersymbol interference. I clo.to. 0.9 Modulo. tecl Co.rrier 0.8 0.7 Q do.to. .,.. 0.6 :s ... 0.5 a"" Figure 4 QPSK Demodulator ~ 0.4 and comparators and clocked latches recover the 0.3 data. The latches are driven by a symbol timing 0.2 recovery circuit which reconstructs the transmit data clock at the proper phase so that the sampling time 0.1 is correct. 0.0 0 50 100 150 200 250 While QPSK can theoretically achieve a spectral Frequency (kHz) efficiency of 2 bits/sec/Hz, this is not achievable in Figure 5 Raised Cosine Nyquist Filters· practice since a brick wall filter is not realizable. The steepness of the filter determines the spectral efficiency. Steep filters ring, and as the spectral When we speak of the channel filter, we mean the efficiency of QPSK is raised by sharpening the filter function of the complete channel. This filters, the ringing of the individual data pulses includes the transmit baseband lowpass filter, any increases. This ringing can cause the pulses to transmit RF filtering, receiver preselection and IF interfere with each other (intersymbol interference) filtering, and receive baseband lowpass filtering. It unless the ringing is controlled. If the filtering is also includes the effect of the transmission path done properly, each individual pulse waveform, which might have a non-flat response (or selective except for the pulse being detected, will pass through fade). Typically, all of the intended filtering is zero at the instant the receiver samples the designed into the system at IF or baseband, and all waveform. Nyquist filters provide the desired other circuitry is made wideband. The most precise characteristics. A commonly used class of Nyquist control over filtering is achieved with baseband filters are the "raised cosine" filters which have a lowpass filtering. It is much easier to make a frequency response as described in Eq. I , and shown precision 125 kHz lowpass filter than a precise 250 in Fig. 5. The alpha term determines the fractional kHz IF filter. 52- 1990 NCfA TECHNICAL PAPERS QPSK Spectral Efficiency = ~ bits I Hz (2) 1+a As alpha is reduced, spectral efficiency of QPSK is improved. Equation 2 shows the relationship between spectral efficiency and alpha. As alpha is reduced, the data waveform will have more ringing and the "eye" pattern will become more complex. The eye pattern is observed at the point where the analog waveform is sampled to recover the data. In the QPSK decoder block diagram (Fig. 4), the eye monitor point is shown for the I channel, and is just after the baseband lowpass filter. In Fig. 6 we see some examples of eye patterns for alpha=0.3, Figure 6c Eye Pattern, Alpha = 0.3 alpha=0.7, and alpha=l.O. The eye patterns become more complex as alpha is lowered and spectral efficiency is improved. More accuracy is required in implementation of low alpha systems. Any error in amplitude or phase linearity will rapidly degrade a complex eye. The eye will appear to close. Fig. 5d shows an alpha = 0.3 eye with a lot of closure due to poor filtering. Partial eye closure due to imperfect filtering leaves less margin against noise and interference. + • -- . - -·~·-r----··· ---. r- Figure 6a Eye Pattern, Alpha = 1.0 Figure 6d Alpha = 0.3, Poor Filter Accuracy For optimal performance over a noisy channel, the amplitude portion of the filtering should be partitioned in equal portions between the transmitter I Figure 6b Eye Pattern, Alpha = 0.